Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which brightness of a pixel is based on the intensity of the echo return. The echo return signals are a combination of fundamental and harmonic signal components, the former being direct echoes of the transmitted pulse and the latter being generated in a nonlinear medium, such as tissue, from finite-amplitude ultrasound propagation.
Until recently, medical ultrasound was considered a linear imaging process. The generation of harmonic signals along the wave propagation path was perceived to be a nuisance. Their effect was limited to creating difficulties for acoustic power measurements and generating an abnormal energy absorption pattern. Recently however, tissue harmonic imaging has received much attention for its ability to provide improved image quality in otherwise difficult imaging situations. Often an image is obscured by clutter which originates from low-amplitude, low-frequency waves bouncing between the transducer and subcutaneous layers. Since these clutter components do not create higher harmonics (at least not to a significant level), they do not appear in images where the fundamental frequency is filtered out.
One known implementation for performing tissue harmonic imaging uses a bandpass filter to separate the second harmonic from the fundamental frequency. Assuming a transmitted signal centered at f.sub.0, the receive filter is centered at 2f.sub.0. This method, while improving the image quality, is accompanied by significant implementation difficulties. Particular care must be taken to design a transmit waveform which does not create significant (linear) signal components in the range of the receive filter. This design requirement can be effectively addressed with multilevel pulsers. The biggest challenge in second harmonic imaging is the bandwidth requirement. Assuming the transmitted signal to have a frequency f.sub.0 with bandwidth B (i.e., f.sub.0 .+-.B/2), a receive filter covering the frequency range 2f.sub.0 .+-.B is required. Accordingly, the imaging system should provide a passband from f.sub.0 -B/2 to 2f.sub.0 +B. Since most ultrasound transducers are not able to support this bandwidth, significant losses occur. The transmit band is shifted into the lower cutoff region, resulting in a low transmission efficiency. Most of the pulser energy is converted into thermal energy and the achievable acoustic output energy is limited by transducer heating. Similarly, the receive filter is shifted into the upper cutoff region, resulting in a reduced sensitivity. Additionally, the reflected second harmonic echo incurs a higher attenuation due to the frequency-dependent attenuation. These effects combined reduce the sensitivity of second harmonic imaging.
To avoid the problem with transmit energy leaking into the second harmonic, a method of ultrasound imaging has been devised in which two pulses with opposite polarity are transmitted for every ultrasound line and the resulting echo signals are added. The linear signal components cancel out, due to the opposing polarity, whereas the second harmonic signal components are added. In this way the second harmonic and fundamental signal components can be separated without using frequency filters. The bandwidth requirements remain the same, however, since the second harmonic signal components occur in the higher frequency range. Similarly, another method which also uses two transmit pulses sends these transmit pulses with different amplitudes. The echo signals are weighted and subtracted in order to cancel the fundamental signal component.